feat(subscription): 支持调整订阅时长（延长/缩短）

- 将"延长订阅"功能改为"调整订阅"，支持正数延长、负数缩短
- 后端验证：调整天数范围 -36500 到 36500，缩短后剩余天数必须 > 0
- 前端同步更新界面文案和验证逻辑

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/i18n/locales/en.ts

@@ -950,7 +950,7 @@ export default {
       title: 'Subscription Management',
       description: 'Manage user subscriptions and quota limits',
       assignSubscription: 'Assign Subscription',
-      extendSubscription: 'Extend Subscription',
+      adjustSubscription: 'Adjust Subscription',
       revokeSubscription: 'Revoke Subscription',
       allStatus: 'All Status',
       allGroups: 'All Groups',
@@ -965,6 +965,7 @@ export default {
       resetInHoursMinutes: 'Resets in {hours}h {minutes}m',
       resetInDaysHours: 'Resets in {days}d {hours}h',
       daysRemaining: 'days remaining',
+      remainingDays: 'Remaining days',
       noExpiration: 'No expiration',
       status: {
         active: 'Active',
@@ -983,28 +984,32 @@ export default {
         user: 'User',
         group: 'Subscription Group',
         validityDays: 'Validity (Days)',
-        extendDays: 'Extend by (Days)'
+        adjustDays: 'Adjust by (Days)'
       },
       selectUser: 'Select a user',
       selectGroup: 'Select a subscription group',
       groupHint: 'Only groups with subscription billing type are shown',
       validityHint: 'Number of days the subscription will be valid',
-      extendingFor: 'Extending subscription for',
+      adjustingFor: 'Adjusting subscription for',
       currentExpiration: 'Current expiration',
+      adjustDaysPlaceholder: 'Positive to extend, negative to shorten',
+      adjustHint: 'Enter positive number to extend, negative to shorten (remaining days must be > 0)',
       assign: 'Assign',
       assigning: 'Assigning...',
-      extend: 'Extend',
-      extending: 'Extending...',
+      adjust: 'Adjust',
+      adjusting: 'Adjusting...',
       revoke: 'Revoke',
       noSubscriptionsYet: 'No subscriptions yet',
       assignFirstSubscription: 'Assign a subscription to get started.',
       subscriptionAssigned: 'Subscription assigned successfully',
-      subscriptionExtended: 'Subscription extended successfully',
+      subscriptionAdjusted: 'Subscription adjusted successfully',
       subscriptionRevoked: 'Subscription revoked successfully',
       failedToLoad: 'Failed to load subscriptions',
       failedToAssign: 'Failed to assign subscription',
-      failedToExtend: 'Failed to extend subscription',
+      failedToAdjust: 'Failed to adjust subscription',
       failedToRevoke: 'Failed to revoke subscription',
+      adjustWouldExpire: 'Remaining days after adjustment must be greater than 0',
+      adjustOutOfRange: 'Adjustment days must be between -36500 and 36500',
       pleaseSelectUser: 'Please select a user',
       pleaseSelectGroup: 'Please select a group',
       validityDaysRequired: 'Please enter a valid number of days (at least 1)',